1. Field of the Invention
This invention relates to a rotary cylinder dryer for drying tobacco under high humidity conditions and in particular relates to a tobacco drying process in which due regard is given to the gain of filling power which is achieved when the tobacco is dried in a steam atmosphere.
The dryer of the present invention is intended for use with all forms of tobacco leaf but more particularly for the separated midrib in the finely cut or rolled and cut form. The latter is known as cut rolled stem (C.R.S.).
2. Statement of Prior Art
A rotary cylinder dryer of the single cylinder type is described in our UK Patent No. 1,209,929 and the double cylinder type (annular dryer) is described in our UK Patent No. 1,345,373. The cylinder of both these types are heated by steam or high pressure hot water. Rotary cylinder dryers are also known which are heated by other means such as a direct flame or a hot gas.
These dryers are of the type where the heated cylindrical shell provides the majority of the heat for drying by conduction or radiation from the shell to the tobacco and the ventilation air through the cylinder removes the moisture. The air flow is limited from 0.3 to 0.6 m/s to avoid making the tobacco airborne and so the air can only contribute a minority of the drying heat, but very often provides none.
Filling power is the specific volume or inverse of bulk density measured under defined conditions. Typically a 50 g sample, which has been brought to a standard moisture content and temperature, is placed in a 60 mm dia vertical cylinder and subjected to a free falling piston load of 3 kg for 30 seconds before measuring the height of the tobacco and calculating the filling power.
The filling power is usually expressed as cc/gm or ml/g. The filling power gain of a process is the percentage increase in the filling value from before to after the process.
It has been established that drying tobacco in a high humidity atmosphere results in a filling power gain. This has been shown for air duct type dryers such as described in UK Patent application No. 2004999A in which the heat for drying is entirely supplied by the air which carries off the vapor. The circulating air has a wet bulb temperature of at least 65 degrees C. and a dry bulb temperature up to 343 degrees C.
The humidity may be further increased until the wet bulb is 100 degrees C. i.e. the saturation temperature for steam at atmospheric pressure, as described in UK Patent application No. 2 149 897A, in which case dry bulb temperatures of 343 degrees C. to 510 degrees C. are used, i.e. the steam is superheated to provide the heat for drying the tobacco.
Similarly it is established practice to raise the humidity of the ventilating air in a rotary dryer in order to increase the filling power of the tobacco, but this has up to now been confined to modest increases in humidity and the retention of air as a ventilating means.
For tobacco to dry, the vapor pressure at the surface of the tobacco must exceed the vapor pressure in the surrounding atmosphere. If the water in the tobacco was `free` water then the water on the tobacco would start to evaporate when its temperature equalled the saturation temperature of the surrounding atmosphere which is related to the humidity. In fact tobacco is hygroscopic and most of the water is `bound` water so evaporation does not start until the tobacco exceeds the saturation temperature. Bound water is described in "Elements of Chemical Engineering" by Badger and McCabe, second edition, published by McGraw-Hill.
There is thus a warm up period at the start of a drying process when the tobacco is being heated and the vapor pressure within the tobacco is increasing without any loss of moisture. In fact if the atmosphere is saturated there will be condensation on the tobacco and an increase in moisture during this period. It appears that the increase in filling power with high humidity drying is due to the enhanced vapor pressure of the moisture within the cellular structure of the tobacco achieved during the warm up period and sustained during the drying period.
During the curing of tobacco moisture is lost, the tobacco withers and its cellular structure collapses. During processing some moisture is returned to the tobacco to assist in the cutting and rolling processes. In the final drying stage under high humidity conditions the application of heat softens the cell wall structure and the pent up vapor pressure tends to restore the original size, which is retained by subsequent hardening of the structure as moisture and temperature are reduced.
Although there is an increase in volume of the tobacco particles which increases the filling power there is also an increase in the beam strength of the particles which further increases the filling power gain.
In practice high temperatures are not desirable with cut lamina because high temperature causes undesirable darkening of the lamina. High temperatures are not so detrimental to the mid rib so maximum gains are obtained with C.R.S. where the expansion and stretching of the particles can result in a lightening of the product color.
The current practice to achieve maximum filling power when drying C.R.S. is to use a rotary dryer preceded by a separate steaming and moistening process. The means used include a screw conveyor, vibrating conveyor or small rotary cylinder. In each case incorporating steam nozzles to heat the tobacco and water sprays to add permanent moisture.
The aim has been to heat the C.R.S. with saturated steam to above 90 degrees C. and as near 100 degrees C. as possible and to increase the moisture content to around 50 percent. The steaming and moistening process produce C.R.S. with the `pent up` high vapor pressure condition which is then presented to the dryer.
The dryer is generally operating under fairly high humidity conditions because of the high drying load imposed by the added moisture and limited air flow. But this is not necessarily so and in production filling power gains do vary from 20 to 45%.
The object of increasing the moisture content is to increase filling power by restoring the turgor condition of the tobacco. However, this has the undesirable effect of requiring more drying capacity and it has now been found that further increases of tobacco temperature are more beneficial than increases of moisture content and require less drying capacity.
The saturated water vapor pressure for temperatures around boiling point are:
Vapor pressure, bar: 0.5, 0.7, 1.0, Saturated temp., degrees: C. 81, 90, 100.
It is clear that an increase of only 19 degrees C. from 81 degrees C. to 100 degrees C. has doubled the vapor pressure. However, the moisture in the tobacco is largely `bound` moisture which exerts a vapor pressure less than that of `free` moisture at the same temperature, so that the tobacco must be heated above 100 degrees C. to achieve the max vapor pressure of 1,0 bar at atmospheric pressure.
The temperature to which the tobacco must be heated depends on its percentage moisture content. For example at 50 percent there is free moisture so the temperature is 100.degree. C., whereas at 35 and 12 percent the temperature is typically 104.degree. C. and 114.degree. C. respectively for cut stem.
The purpose of the preparatory heating process is to raise the tobacco above the 100 degrees C. barrier set by a saturated mixture of steam and air at atmospheric pressure. This may be achieved by heating with superheated steam as described in UK Patent application No. 2 138 666 in which filling power gains in excess of 50% are claimed.
It has been found that the filling power gain is only retained at low moisture and temperatures, both of which conditions tend to harden the tobacco structure. Accordingly it is desirable to maintain the high humidity condition throughout the drying process to prevent any loss of filling power gain.
The maximum humidity can be retained in a rotary dryer by providing a 100 percent steam atmosphere within the drier, and drying without air.
A typical cylinder temperature of a steam heated drier is 160 degrees C. So with a steam atmosphere and a product temperature of near 100 degrees C. the temperature difference is only 60 degrees C. Temperature difference determines the drying capacity, so a high product temperature reduces the drying capacity.